1. Field of the Invention
This invention concerns a light emitting semiconductor device, such as a light emitting diode device, and a light emitting semiconductor device array.
2. Description of the Related Art
Light emitting diode (LED) devices have been widely employed as display devices for some time because the light which they emit is brilliant, the drive voltage is low, and the peripheral circuitry is simple.
Conventionally, the LED arrays used in photo printers are such as are disclosed, for example, in Hikari Purinta Sekkei (Photo Printer Design), pp 121-126, ed. by Y. Takekida, Torikeppusu KK, Oct. 31, 1985. Conventionally these have been fabricated by selectively diffusing zinc on a substrate made by epitaxial growth of GaAsP on a GaAs substrate (hereinafter called a GaAsP.sup.- substrate).
FIG. 14 is a diagram which models conventional LED fabrication. This LED comprises an n-type GaAs substrate 11, an n-type GaAsP epitaxial layer 12 wherein Te is doped and epitaxially grown on the n-type GaAs substrate 11, a p-type GaAsP epitaxial region 13 formed by diffusing Zn, a SiN insulating film 14 which functions as a mask for Zn diffusion, an Al electrode 15, and an Au--Ge electrode 16. This LED is structured by the formation of a pn junction by diffusing zinc, which is a p-type impurity, in an n-type GaAsP substrate. Junctions having this structure are commonly called homo junctions.
LEDs structured in this way provide the advantages of requiring comparatively few fabrication steps and of being easy to fabricate. In such LEDs as these, the minority carriers injected through the junction recombine with the majority carriers and emit light. The wavelength of this emitted light corresponds to the energy band gap of the substrate semiconductor. For this reason, the light emitted is affected by large light absorption in the p-type region as it passes through, wherefore the light emission efficiency is not high, which is a problem.
In distinction to LEDs based on the homo junction, as described above, there are LEDs which use a pn junction (hereinafter called a hetero junction) formed by joining a different crystal, such as is set forth, for example, in Hakko Daiodo (Light-Emitting Diodes), pp. 27-31, Okuno, Sangyou Tosho KK, Jan. 20, 1994. As will be described subsequently, the light emission efficiency of an LED can be made higher with a hetero junction than with a homo junction.
FIG. 15(A) is a diagram of the structure of an LED using the hetero structure, while FIG. 15(B) is a diagram of an example of an energy band gap therein. FIG. 15 is a diagram of an example of an LED commonly called a single hetero structure (SH structure).
The LED having the single hetero structure shown in FIG. 15(A) is structured with a p-type Al.sub.0.35 Ga.sub.0.65 As layer 21 epitaxially grown on a p-type GaAs substrate 20, with an Al.sub.0.65 Ga.sub.0.35 As layer 22 grown epitaxially on top of that. Reference numerals 23 and 24 show Au electrodes, respectively.
With this structure, as shown in FIG. 15(B), the positive holes injected through the junction are blocked from diffusing by the energy barrier at the hetero junction interface, whereupon the proportion that recombines increases. The wavelength of the light emitted, moreover, corresponds to the energy band gap of the Al.sub.0.35 Ga.sub.0.65 As layer 21, while the n-type Al.sub.0.65 Ga.sub.0.35 As layer 22 that forms a light window is larger than the energy band gap of the Al.sub.0.35 Ga.sub.0.65 As layer 21, so the emitted light is not absorbed in the semiconductor region 22 that constitutes the window. Accordingly, the light emission efficiency increases.
LED arrays in which the LEDs described above are integrated are used as light sources in LED printers, for example. When an LED array is fabricated by homo junctions, pn junction arrays can be easily fabricated by selective diffusion in which the diffusion into the semiconductor is performed through the diffusion mask openings. The fabrication of LED arrays by this selective diffusion is easy, and it is possible to fabricate a super-high-density 1200 dpi LED array.
With a conventional hetero structured LED such as this, a semiconductor layer having an energy band gap larger than the energy of the light forms a window, so there is no light absorption, and the efficiency with which the light is drawn to the outside is enhanced. On the other hand, with an LED array using LEDs of this structure, it is necessary to provide device separation between the devices. For that purpose, the elements are separated, for example, by mesa etching. This places limitations on the integration density of the LED array.
Thus, in the related art, there is no technology for fabricating LED arrays that exhibit high light emission efficiency and are capable of super-high-density integration.
An object of the present invention is to provide both a light emitting semiconductor device and a light emitting semiconductor device array that exhibit high light emission efficiency and wherewith high densities can be achieved.
Another object of the present invention is to provide both a light emitting semiconductor device and a light emitting semiconductor device array that can be mass-produced at low cost and high yield.